1. Field of the Invention
The present invention is broadly concerned with Hg-based superconducting cuprate films, and methods and apparatus for preparing the same. More particularly, the present invention relates to the "Fast Temperature Ramping Hg-Vapor Annealing" (FRTA) method for fabricating Hg-based superconducting cuprate films, an apparatus for transforming a superconducting film precursor into a superconductor by contacting the precursor with a metal cation-containing vapor, and resultant superconducting films.
2. Description of the Prior Art
Superconductivity, a phenomenon occurring at very low temperatures in many electrical conductors, occurs when electrons responsible for conduction undergo a collective transition to an ordered state. Many unique and remarkable properties result from this transition, including the vanishing of resistance to the flow of electric current, the appearance of a large diamagnetism and other unusual magnetic effects, substantial alteration of many thermal properties, and the occurrence of quantum effects otherwise observable only at the atomic and subatomic levels.
Superconductivity was discovered in 1911 by H. Kamerlingh Onnes while studying the variation with temperature of the electrical resistance of mercury within a few degrees of absolute zero. He observed that the resistance dropped sharply to an unmeasurably small value at a temperature of 4.2.degree.K. The temperature at which this transition occurs is called the critical temperature (T.sub.c).
Until recently, all known superconducting materials attained superconductivity at very low temperatures on the order of 4.degree.-20.degree. K. Such low temperatures had to be reached by evaporating liquid helium, the only substance that remains liquid at temperatures approaching absolute zero. The few sources of helium in nature and its expensive processing make it a costly cryogenic fluid. Thus, there is a need for materials which are superconductive at temperatures much higher than absolute zero.
Recently, superconductivity has been discovered in Hg-based cuprates (HgBa.sub.2 Ca.sub.n-1 Cu.sub.n-1 O.sub.2n+2+.delta. ; n=1, 2, or 3), which raised the record for T.sub.c to 135.degree. K. Consequently, many efforts have been devoted to the synthesis of high-quality Hg-based cuprate thin films. However, due to the high volatility and high toxicity of Hg and Hg-based compounds, the synthesis of such films is extremely difficult. In spite of these difficulties, advances have been achieved recently in growth of epitaxial Hg-based cuprate thin films.
Only (100) SrTiO.sub.3 substrates have been used previously in the growth of the Hg-based cuprate thin films. However, while SrTiO.sub.3 substrates have many superior properties including chemical stability in Hg vapor, they are expensive, are available only in small size, and have poor microwave properties such as high dielectric constant and high microwave tangent loss. It is thus necessary to find other lower-cost, large-size, and microwave-compatible substrates that can be used in the growth of Hg-based cuprate thin films.
One such substrate, LaAlO.sub.3, is available in large size (e.g., a wafer having a diameter greater than 3 inches) at much lower cost than SrTiO.sub.3, and has excellent microwave properties. Hg-based cuprate films on LaAlO.sub.3 substrates have T.sub.c 's nearly 20.degree. K. lower than T.sub.c 's obtained with Hg-based cuprate films on SrTiO.sub.3 substrates, and have critical current densities (J.sub.c 's) more than two orders of magnitude lower than J.sub.c 's obtained with Hg-based cuprate films on SrTiO.sub.3 substrates. Since the chemical stability of LaAlO.sub.3 is not as good as that of SrTiO.sub.3 at high temperatures in the presence of Hg vapor, chemical reactions and interdiffusion near the film/substrate interface seriously degrade the superconducting properties of the films. Thus, until now, only poor-quality Hg-based cuprate thin films have been obtained on LaAlO.sub.3 substrates.
In the conventional annealing method previously used for Hg-based cuprates, the sample temperature is increased slowly (e.g., 4-6 hours) to the annealing temperature (e.g., 780.degree.-860.degree. C.) in order to maintain phase equilibrium between a precursor film and an unreacted HgO+Ba.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.y pellet which are sealed together in an evacuated quartz tube. Since HgO decomposes at around 500.degree. C., two problems arise using this slow-heating cycle. First, at temperatures above 500.degree. C., Hg.sup.+2 begins to react with Ca to form CaHgO.sub.2, a compound which seriously degrades the superconducting properties of the sample. Second, the high-temperature processing time (e.g., 500.degree. C. to annealing temperature) increases the problem of film/substrate interface chemical reaction and interdiffusion. Consequently, only HgBa.sub.2 CaCu.sub.2 O.sub.6+.delta. (i.e., Hg-1212) thin films have been obtained with the conventional method. No success has been reported with HgBa.sub.2 Ca.sub.2 Cu.sub.3 O.sub.8+.delta. (i.e., Hg-1223) thin films because a relatively high annealing temperature is needed, making interfacing and CaHgO.sub.2 problems more severe. Furthermore, only a SrTiO.sub.3 substrate has been used in the conventional annealing method, with the resultant films having poor surface morphology.
Moreover, the conventional annealing method has not been successful in generating a high-quality double-sided superconducting film (a double-sided superconducting film is a composite comprised of a double-sided substrate having a superconducting film on each side thereof). Such double-sided superconducting films have many applications in microelectronics (e.g., superconducting quantum interference devices and microwave resonators). A double-sided superconducting film is conventionally prepared by depositing and annealing the first side of the film, and subsequently depositing and annealing the second side of the film.
Since most in-situ deposition methods utilize high deposition temperatures of approximately 750.degree.-800.degree. C., the superconducting properties of the first side of the film degrade when the first side is again heated during annealing of the second side of the film. Therefore, it is difficult to produce double-sided superconducting films with sides having identical physical properties. Differences between the first and second sides in double-sided superconducting films have hindered the application of high-temperature superconductors in the microelectronics industry.